Photovoltaic energy is becoming a very significant power source for several reasons. Fossil fuels are becoming scarcer, and hence, more expensive, everyday. Furthermore, the burning of fossil fuels releases pollutants, including greenhouse gases, which contribute to problems of global warming. Recent events have raised questions as to the safety and cost-effectiveness of nuclear power, and for these reasons such traditional energy sources are becoming less attractive. Photovoltaic energy, on the other hand, is inherently non-polluting, safe and silent. In addition, recent advances in photovoltaic energy have significantly increased the efficiency, and decreased the cost of such devices. For example, it is now possible to manufacture large area, thin film silicon and/or germanium alloy materials which manifest electrical, optical, chemical and physical properties equivalent, and in many instances superior to, their single crystalline counterparts. Layers of such alloys can be economically deposited at high speed over relatively large areas and in a variety of stacked configurations. Such alloys readily lend themselves to the manufacture of low cost photovoltaic devices. Some materials having particular utility in the fabrication of photovoltaic devices are described in U.S. Pat. Nos. 4,226,898 and 4,217,374, the disclosures of which are incorporated herein by reference. It will be appreciated that other thin film photovoltaic materials such as CuInSe.sub.2, CdS materials and the like also have potential uses as photovoltaic devices.
As will be described in greater detail herein below, it is generally preferable to encapsulate or otherwise protect photovoltaic devices from cuts, abrasions and other ambient conditions, particularly when they are employed as power generating devices. The power producing photovoltaic installation will, of necessity, generate high electrical currents and/or voltages, and encapsulation provides the additional advantage of minimizing electrical shock hazards. Glass has been used as a protective material; however, glass is heavy and brittle, and can present a safety hazard if broken. Polymeric materials are lightweight and flexible; but such materials tend to be soft and easily cut; furthermore, their flexibility can cause them to be relatively thin in the region of thick structures such as bus bars, gridwires and the like, and this thinness will decrease cut resistance of the laminate. Various safety standards have been promulgated relative to photovoltaic devices concerning shock hazards resultant from inadvertent contact with the device, and in accord therewith, it is necessary that photovoltaic devices be resistant to cutting, penetration or abrasion. It will be appreciated that there is a need for encapsulant material which is capable of safely protecting a photovoltaic device. This encapsulant should also be low in cost, lightweight and not compromise the efficiency of the photovoltaic device.
The Underwriters Laboratory (UL) is a certifying organization which develops safety standards for a variety of products including photovoltaic devices. In order to better illustrate the operation and advantages of the encapsulant design of the present invention, the Underwriters Laboratories specifications will be enumerated in the following paragraphs. In publication UL 1703 entitled "Flat-Plate Photovoltaic Modules And Panels", Section 23, Underwriters Laboratories set forth the specifications for a "cut test" that must be met by flat panel photovoltaic modules, such as those for which the encapsulant of the instant invention was developed. That section reads as follows:
23.1 A module or panel shall be capable of withstanding the application of a sharp object drawn across its superstrate and substrate surfaces without creating a risk of electric shock. PA1 23.2 The module or panel is to be positioned in a horizontal plane with the surface to be tested facing upward. The tool illustrated in FIG. 23.1 is to be placed on the surface for 1 minute, and then drawn across both the front and rear surfaces of the module or panel at a speed of 6.+-.1.2 inches per second. PA1 23.2 A risk of electric shock is considered to exist if either the blade of the tool contacts a part involving risk of electric shock, or if such a part is rendered accessible (transitory or permanent) as a result of the placement of the blade on or the drawing of the blade across the surface.
The Section 23 specification can be better understood by turning now to FIGS. 3-6 which correspond to FIG. 23.1 of the UL specification. In FIG. 3 the cut test tool is illustrated generally by the reference numeral 50. Tool 50 includes a generally rectangularly shaped frame formed by front brace 52a, back brace 52b, longitudinal side braces 52c and 52d, and four wheels 54 rotatably secured adjacent the ends of the longitudinal braces by bearing screws 56. A hook 58 secured to the back brace 52b provides means by which the tool can be rolled across the light incident surface of a photovoltaic module disposed therebelow. A rotatable support member 60 extends across and is affixed between the longitudinal braces 52c and 52d by elongated dowel pin 62. Secured to the central section of the support member 60 is an elongated rod 64 to which is affixed a screw 66 attaching a relatively sharply tipped instrumentality, such as a carbon steel strip (hacksaw blade) 68 and weighted blocks 70. For the purpose of specificity and referring to FIGS. 3-6, dimension A is about 87/8 inches from the axis to the center of the weight; dimension b is about 65/8 inches from the axis to the test point; dimension C is about 0.025 inches and represents the thickness of the testing tip; and reference letter Q represents the total force exerted at the test point, i.e, at least two pounds.
The improved, cut resistant, composite encapsulant of the instant invention is specifically adapted to meet the Underwriters Laboratories specifications by withstanding at least two pounds of force when the test tool 50 is pulled across the surface thereof, while preventing the potential electrical shocks initiated thereby. Furthermore, the composite encapsulant is lightweight, low in cost, and highly transparent. These as well as other advantages of the instant invention will be readily apparent from the drawings, the detailed description thereof and the claims which follow hereafter.